The present invention relates to an optical recording method and an optical recording medium.
Along with remarkable increase of the quantity of information in recent years, an information recording with high density and large capacity has been more and more required. An optical disc has advantageous features of high recording density, large capacity and excellent random accessibility. Further, since recording/reading out is conducted in a contactless state with an optical head, the recording medium less suffers from damages and is highly resistant to dusts or scratches to a disc surface and excellent in long-term reliability. The optical disc having such features can provide recording media just capable of satisfying social demands, and it is expected that the demand therefore will be further increased.
At present, various recording systems have been proposed for the optical disc. In the field of external memory devices for computers, a system referred to as a land recording type is predominant, in which guide grooves for tracking are disposed and information is recorded in flat areas (land) between the grooves.
For information recording/reading out, a semiconductor laser beam focused into a minute size by an objective lens is used usually. The minimum spot diameter (dmin) of the focused laser beam is represented by the following equation: EQU dmin=k.multidot..lambda./NA (1)
wherein .lambda. is a wavelength of a laser beam used, NA is a numerical aperture of an objective lens, and k is a constant determined by the aperture shape of the lens and intensity distribution of incident optical rays.
Semiconductor lasers put to practical use at present for optical disc systems include two types having wave length regions of 825 nm and 780 nm. Further, objective lenses usually have a numerical aperture of not more than 0.55.
The recording density of an optical disc is restricted significantly by the laser beam diameter drain. The minimum mark length capable of recording/reading out is substantially determined depending on dmin excepting for a case of using special technique such as super-resolution. Accordingly, it is possible to increase both the linear recording density and the radial recording density by decreasing dmin.
For decreasing dmin, it is at first considered to use a lens of a large numerical aperture. However, as NA is increased, the focal depth becomes smaller as shown in the equation (2) and an allowance for the inclination of the disc is reduced abruptly and, accordingly, this means it is difficult for a high NA lens to be adopted for a practical system. EQU Focal depth=.lambda./(NA).sup.2 ( 2)
Further, in a disc of a substrate incident type, since there is a risk that an objective lens is brought into contact with a disc as NA is increased, the upper limit of NA is about 0.70 for a substrate of 1.2 mm thickness used at present.
In view of the above, it is effective for attaining high density recording to use a laser of shorter wavelength as a light source. If the wavelength of the laser beam is not more than 500 nm, it is theoretically possible to increase the recording capacity to more than ten times of existent optical discs, by combination with a mark length recording system (FWM system) or MCAV (Modified Constant Angular Velocity) system.
However, in a case of using a short wavelength laser, although the linear recording density and the radial recording density of the optical disc could theoretically be increased each to an equal high density, the recording density in the radial direction is not so high as that in the linear direction.
In both of the land recording system and the groove recording system, an amount of light incident to a signal detector decreases by so much as a laser beam is diffracted and scattered by lands or grooves, and a signal intensity decreases. If a track pitch is narrowed for increasing the radial density, the amount of diffracted and scattered is naturally increased to result in remarkable reduction in the amount of reflected light.
For instance, in the land recording system, although the guide groove itself can be narrowed to some extent, there is also a limit for narrowing the groove while keeping a depth capable of obtaining a sufficient tracking signal as it is. Further, if the groove, is deep and narrow, since the gradient inclination of a slope is abrupt, it is difficult to achieve homogeneous covering of a recording layer.
Further, since an edge portion between the groove and the land is not smooth but microscopically uneven, this causes noises. Then, the effect on the signal tends to increase not gradually but rather abruptly at a portion in which a track pitch is narrowed to some extent. This is a limit in narrowing the track pitch and it is solely determined depending on the laser wavelength providing that NA is constant.
For instance, at NA of 0.55, since the limit for the track pitch at a wavelength of 780 nm is about 1.35 .mu.m in view of a practical level, a lower limit for the track pitch at a wavelength of 500 nm is 1.35 (.mu.m).times.500 (nm)/780 (nm)=0.87 (.mu.m), that is, about 0.9 .mu.m in a case where guide grooves are present.
Further, when a short wavelength laser beam is used, there are the following problems inherent to a short wavelength region.
This is, at first, lowering of the photosensitivity in a light detector. In a light detection device and photodiode used at present for an optical disc drive, light is detected by measuring electric current caused when light excites electrons in the vicinity of pn Junction into a conduction band and electrons in the band move through the pn junction. Since light absorption coefficient of a Si semiconductor constituting a photodiode increases toward the short wavelength, light at short wavelength is absorbed near the surface of the photodiode and less reaches to the vicinity of the pn junction. As a result, since the numerical electrons excited in the vicinity of the pn junction decreases, the photosensitivity is lowered. For instance, the sensitivity at 500 nm lowers to about 1/2 as compared with that at a wavelength of 780 nm. Namely, the signal intensity is reduced to about one-half at 500 nm even for a signal of an identical reflectivity.
On the other hand, as compared with the reduction of the signal intensity, noises are less reduced. Noises in the optical disc system include noises generated from a medium, as well as noises contained in the oscillated laser beam itself and system noises generated in the detection system.
Among them, since the system noise is constant irrespective of the signal intensity, the entire noise intensity is increased as compared with the signal intensity to lower C/N. Accordingly, to increase the sensitivity in the short wavelength is a subject for the detector and to increase the signal intensity and decrease noise is a subject for the optical disc.
Further, there is a problem of birefringence in the substrate. The birefringence in the substrate causes reduction of the signal intensity and increase of the noises. Particularly, in a magneto-optical disc, since recording signals are read out by detecting the rotation of the plane of polarization, the birefringence of the substrate gives a direct effect on the characteristics of the readout signals. For instance, it brings about reduction of the signal intensity and increase of the noises. Further, since the birefringence causes aberration, the laser beam can not be focused on the recording layer to the diffraction limit shown by the equation (1) and this also causes reduction of the signal intensity or gives an effect, for example, on jitter and crosstalk. Furthermore, the effect of the birefringence becomes more remarkable as the laser wavelength becomes shorter.
The present inventors have found that by recording information on a flag area not having a guide groove, the problem discussed above can be overcome, and have accomplished the present invention based on the finding.